The foundations of the endocrine system are the hormones and glands. Hormones are the body's chemical messegers that transfer information and instructions from one set of cells to another. Many different hormones move through the bloodstream, but each type of hormone is designed to affect only certain cells.
A gland is a group of cells that produces and secretes, or gives off, chemicals. A gland selects and removes materials from the blood, processes them, and secretes the finished chemical product for use somewhere in the body. Some types of glands release their secretions in specific areas. Exocrine glands such as the sweat and salivary glands release secretions in the skin or inside the mouth. Endocrine glands, on the other hand, release more than 20 major hormones directly into the bloodstream where they can be transported to cells in other parts of the body.
The major glands that make up the human endocrine system include the:
-hypothalamus
-pituitary gland
-thyroid
-parathyroids
-adrenal glands
-pineal body
-reproductive glands (which include the ovaries and testes)
The hypothalamus is a collection of specialized cells that is located in the lower central part of the brain, is the main link between the endocrine and nervous systems. Nerve cells in the hypothalamus control the pituitary gland by producing chemicals that either stimulate or suppress hormone secretions from the pituitary.
Although it is no bigger than a pea, the pituitary gland, located at the base of the brain just beneath the hypothalamus, is considered the most important part of the endocrine system. It's often called the "master gland" because it makes hormones that control several other endocrine glands. The production and secretion of pituitary hormones can be influenced by factors such as emotions and changes in the seasons. To accomplish this, the hypothalamus provides information sensed by the brain (such as environmental temperature, light exposure patterns, and feelings) to the pituitary.
The tiny pituitary is divided into two parts: the anterior lobe and the posterior lobe. The anterior lobe regulates the activity of the thyroid, adrenals, and reproductive glands. The anterior lobe produces hormones such as:
growth hormone, which stimulates the growth of bone and other body tissues and plays a role in the body's handling of nutrients and minerals
prolactin which activates milk production in women who are breastfeeding
thyrotropin which stimulates the thyroid gland to produce thyroid hormones
corticotropin which stimulates the adrenal gland to produce certain hormones
The pituitary also secretes endorphins chemicals that act on the nervous system and reduce feelings of pain. In addition, the pituitary secretes hormones that signal the reproductive organs to make sex hormones. The pituitary gland also controls ovulation and the menstrual cycle in women.
The posterior lobe of the pituitary releases antidiuretic hormone, which helps control the balance of water in the body. The posterior lobe also produces oxytocin which triggers the contractions of the uterus in a woman having a baby.
The thyroid located in the front part of the lower neck, is shaped like a bow tie or butterfly and produces the thyroid hormones thyroxine and triiodothyronine.These hormones control the rate at which cells burn fuels from food to produce energy. The production and release of thyroid hormones is controlled by thyrotropin which is secreted by the pituitary gland. The more thyroid hormone there is in a person's bloodstream, the faster chemical reactions occur in the body.
Thyroid hormones are very inportant in kids and teens because they help develop the brain and nervous system and also they help bones develop and grow.
Attached to the thyroid are four tiny glands that function together called the parathyroids .They release parathyroid hormone, which regulates the level of calcium in the blood with the help of calcitonin which is produced in the thyroid.
The body also has two triangular adrenal glands, one on top of each kidney. The adrenal glands have two parts, each of which produces a set of hormones and has a different function. The outer part which is called the adrenal cortex produces hormones called corticosteroids that influence or regulate salt and water balance in the body, the body's response to stress, metabolism, the immune system, and sexual development and function. The inner part, the adrenal medulla, produces catecholamines,such as epinephrine.Also called adrenaline, epinephrine increases blood pressure and heart rate when the body experiences stress.
The pineal body, also called the pineal gland, is located in the middle of the brain. It secretes melatonin a hormone that may help regulate when you sleep at night and when you wake in the morning.
The gonads are the main source of sex hormones. Most people don't realize it, but both guys and girls have gonads. In guys the male gonads, or testes are located in the scrotum. They secrete hormones called androgens the most important of which is testosterone. These hormones tell a guy's body when it's time to make the changes associated with puberty, like penis and height growth, deepening voice, and growth in facial and pubic hair. Working with hormones from the pituitary gland, testosterone also tells a guy's body when it's time to produce sperm in the testes.
A girl's gonads, the ovaries are located in her pelvis. They produce eggs and secrete the female hormones estrogen and progesterone. Estrogen is involved when a girl begins to go through puberty. During puberty, a girl will experience breast growth, will begin to accumulate body fat around the hips and thighs, and will have a growth spurt. Estrogen and progesterone are also involved in the regulation of a girl's menstrual cycle. These hormones also play a role in pregnancy.
Although the endocrine glands are the body's main hormone producers, some other organs not in the endocrine system - such as the brain, heart, lungs, kidneys, liver, and skin - also produce and release hormones. The pancreas is also part of the body's hormone-secreting system, even though it is also associated with the digestive system because it produces and secretes digestive enzymes. The pancreas produces two important hormones, insulin and glucagon. They work together to maintain a steady level of glucose, or sugar, in the blood and to keep the body supplied with fuel to produce and maintain stores of energy as talked about in the previous unit.
Immune SystemThe immune system is the body's defense against infectious organisms and other invaders. Through a series of steps called the immune response, the immune system attacks organisms and substances that invade our systems and cause disease. The immune system is made up of a network of cells, tissues, and organs that work together to protect the body.
The cells that are part of this defense system are white blood cells or leukocytes. They come in two basic types which combine to seek out and destroy the organisms or substances that cause disease.
Leukocytes are produced or stored in many locations throughout the body, including the thymus, spleen, and bone marrow. For this reason, they are called the lymphoid organs. There are also clumps of lymphoid tissue throughout the body, primarily in the form of lymph nodes, that have the leukocytes.
The leukocytes circulate through the body between the organs and nodes by means of the lymphatic vessels. You can think of the lymphatic vessels as a type of highway between the rest stops that are the lymphoid organs and lymph nodes. Leukocytes can also circulate through the blood vessels. The immune system works in a coordinated manner to monitor the body for germs or substances that might cause problems.
There are two basic types of leukocytes:
The phagocytes which are cells that chew up invading organisms.
The lymphocytes which are cells that allow the body to remember and recognize previous invaders and help the body destroy them.
A number of different cells are considered phagocytes. The most common type is the neutrophil which primarily fights bacteria. So when doctors are worried about a bacterial infection, sometimes they order a blood test to see if a patient has an increased number of neutrophils triggered by the infection. Other types of phagocytes have their own jobs to make sure that the body responds appropriately to a specific type of invader.
There are two kinds of lymphocytes: the B lymphocytes and the T lymphocytes. Lymphocytes start out in the bone marrow and either stay and mature there to become B cells or leave for the thymus gland, where they mature to become T cells. B lymphocytes and T lymphocytes have separate jobs to do: B lymphocytes are like the body's military intelligence system, seeking out their targets and sending defenses to lock onto them. T cells are like the soldiers, destroying the invaders that the intelligence system has identified. Here's how it works.
A foreign substance that invades the body is called an antigen.When an antigen is detected, several types of cells work together to recognize and respond to it. These cells trigger the B lymphocytes to produce antibodies. Antibodies are specialized proteins that lock onto specific antigens. Antibodies and antigens fit together like a key and a lock.
Once the B lymphocytes have produced antibodies, these antibodies continue to exist in a person's body. That means if the same antigen is presented to the immune system again, the antibodies are already there to do their job. That's why if someone gets sick with a certain disease, like chickenpox, that person typically doesn't get sick from it again. This is also why we use immunizations to prevent certain diseases. The immunization introduces the body to the antigen in a way that doesn't make a person sick, but it does allow the body to produce antibodies that will then protect that person from future attack by the germ or substance that produces that particular disease.
Although antibodies can recognize an antigen and lock onto it, they are not capable of destroying it without help. That is the job of the T cells. The T cells are part of the system that destroys antigens that have been tagged by antibodies or cells that have been infected or somehow changed. There are actually T cells that are called "killer cells." T cells are also involved in helping signal other cells like phagocytes to do their jobs.
Antibodies can also neutralize toxins which are poisonous or damaging substances produced by different organisms. Lastly, antibodies can activate a group of proteins called complement that are also part of the immune system. Complement assists in killing bacteria, viruses, or infected cells.
All of these specialized cells and parts of the immune system offer the body protection against disease. This protection is called immunity.
Humans have three types of immunity — innate, adaptive, and passive:
Innate Immunity
Everyone is born with innate or natural immunity, a type of general protection that humans have. Many of the germs that affect other species don't harm us. For example, the viruses that cause leukemia in cats or distemper in dogs don't affect humans. Innate immunity works both ways because some viruses that make humans ill — such as the virus that causes HIV/AIDS — don't make cats or dogs sick either.
Innate immunity also includes the external barriers of the body, like the skin and mucous membranes like those that line the nose, throat, and gastrointestinal tract, which are our first line of defense in preventing diseases from entering the body. If this outer defensive wall is broken like if you get a cut, the skin attempts to heal the break quickly and special immune cells on the skin attack invading germs.
Adaptive Immunity
We also have a second kind of protection called adaptive or active immunity. This type of immunity develops throughout our lives. Adaptive immunity involves the lymphocytes and develops as children and adults are exposed to diseases or immunized against diseases through vaccination.
Passive Immunity
Passive immunity is "borrowed" from another source and it lasts for a short time. For example, antibodies in a mother's breast milk provide an infant with temporary immunity to diseases that the mother has been exposed to. This can help protect the infant against infection during the early years of childhood.
Everyone's immune system is different. Some people never seem to get infections, whereas others seem to be sick all the time. As people get older, they usually become immune to more germs as the immune system comes into contact with more and more of them. That's why adults and teens tend to get fewer colds than kids — their bodies have learned to recognize and immediately attack many of the viruses that cause colds.
